The present invention generally relates to radiation absorptive coatings or substrates for providing isolation between RF radiating and receiving antennas and, more particularly, an improved lightweight coating or composite for this purpose.
Platforms employing RF radiating and receiving antennas use various strategies to isolate the antennas from each other, including the use of absorptive or other coatings on the platform surface. These coatings are designed to reduce or eliminate the propagation of RF energy from one antenna to its neighbors.
Although the present invention is not limited to such application, the problem addressed by the invention may be better understood by referring to FIG. 1, which is a highly schematic representation of a dummy or decoy 10. The decoy 10 includes a receiving antenna 12 which receives a radar signal 14 and which is coupled through a signal processor 16 to a radiating or transmitting antenna 18. The system operates such that when a radar signal is received, transmitting antenna 18 transmits a signal 20 designed to falsely indicate to the radar receiver that the radar return is from an actual target. The receiving and transmitting antennas 12 and 18 are often close together on this and on like platforms and feedback in the form of surface wave energy can impair the system operation.
Currently, the aforementioned surface wave energy, which, as stated, produce unwanted coupling between adjacent antennas, are attenuated by use of composites of ferromagnetic material in a polymer matrix. The composite material commonly used for antenna isolation is MagRAM (magnetic radar absorbing material), a heavy material whose frequency absorption is flat. Such a composite is indicated schematically by composite 22 located between antennas 12 and 18. The amount of absorption by the composite is proportional to the density of magnetic material in the composite and the thickness of the composite and, since magnetic material is heavy, there is a weight penalty to pay. This is an obvious disadvantage in, e.g., a decoy or dummy missile. Considering some patents of interest in the broad field of electrical shielding, U.S. Pat. No. 5,827,997 to Chung et al discloses metal filaments used in a composite for electromagnetic interference (EMI) shielding fabricated by forming a dry mixture of polymer powder and filler in a steel mold. U.S. Pat. No. 5,661,484 to Shumaker et al discloses an artificial dielectric radar absorbing material employing both relatively resistive and conductive filaments which permit frequency dependent, complex permittivities of materials to be produced by the proper selection of dipoles. The lengths of these conductive filaments are less than one half the wavelength of the median frequency of the incident energy in the frequency band to be absorbed.
U.S. Pat. No. 5,298,903 to Janos discloses a synthetic dielectric material for RF ohmic heating using metallic conducting particles of specified shapes and dimensions embedded in a dielectric slab. This heating occurs within the volume of the material in the form of power loss when the phase difference between the conduction current and internal electric field is correspondingly small.
Patents of even more general interest include U.S. Pat. No. 5,104,580 to Henry et al, which discloses a conductive composite polymer film and a manufacturing process therefor which provides for homogeneous placement of conductors in the polymer film to reduce the percolation threshold. U.S. Pat No. 6,013,206 to Price et al discloses formation and metallization of high-aspect lipid microtubules. U.S. Pat. No. 5,203,911 to Sricharoenchaikit et al discloses a controlled electroless plating method wherein the plating thickness on microtubules is controlled through a slow rate of deposition. The general relevance of these patents will become more relevant from the discussions below.
In accordance with the invention, a lightweight coating composite is provided which has dielectric properties which either absorb or xe2x80x9cshedxe2x80x9d RF energy traveling along the surface of an antenna platform to prevent one antenna on the platform from coupling with a neighboring antenna on the platform and thereby interfering with the sensitivity thereof.
In accordance with a first aspect of the invention, there is provided a coating composite for a platform surface of an antenna array for, when applied to the platform, providing isolation of radiating and receiving antennas of the array, the coating composite comprising a plurality of conductively coated elongate tubes dispersed in an insulating polymer matrix at a volume loading density approaching that at which the composite begins to conduct electrically over macroscopic distances.
Preferably, the tubes comprise microtubules comprised of biologically-derived, high-aspect rod-shaped particles of microscopic dimensions having an electroless plated conductive coating thereon. Advantageously, the conductively coated elongate tubes have a metal coating. In a beneficial implementation, the metal of said metal coating is selected from the group consisting of nickel and copper.
Preferably, the volume loading density is less than 20%.
In accordance with a further aspect of the invention, there is provided a covering composite for an antenna platform of an antenna array for providing isolation of radiating and receiving antennas of the array, the covering composite comprising a polymer matrix and a plurality of conductive microtubules dispersed within said matrix, the composite having a percolation threshold and the microtubules being dispersed at a volume loading density expressed as the percentage of the volume of the microtubules with respect to the volume of the polymer matrix of no greater than (Xxe2x88x921)% where X% is the volume loading density corresponding to percolation threshold.
Preferably, the microtubules comprise biologically-derived, high-aspect rod-shaped particles of microscopic dimensions having an electroless plated conductive coating thereon. Advantageously, the conductively coated elongate tubes have a metal coating and, in a preferred implementation, the metal coating is selected from the group consisting of nickel and copper.
Preferably, the percentage is less than 20%.
In accordance with yet another aspect of the invention, there is provided, in an antenna platform including antenna array comprising at least one RF radiating antenna and at least one RF receiving antenna separated from said RF radiating antenna so as to define a space therebetween, a composite disposed in the space between said at least one radiating antenna and said at least one receiving antenna for providing electrical absorption of RF energy so as to provide isolation between the antennas, the composite comprising a plurality of conductively coated insulating tubes dispersed in an insulating polymer matrix.
In a preferred embodiment, the composite has a percolation threshold and the tubes are dispersed in the polymer matrix at a volume loading density expressed as a percentage of the volume of the tubes to the volume of the polymer matrix which is close to that corresponding to said percolation threshold. Advantageously, said volume loading density is no greater than (Xxe2x88x921)% wherein X% is the volume loading density corresponding to the percolation threshold. Preferably, the percentage is less than 20%.
As with the other aspects of the invention, the tubes preferably comprise microtubules comprised of biologically-derived, high-aspect rod-shaped particles of microscopic dimensions having an electroless plated conductive coating thereon. The conductively coated tubes preferably have a metal coating and, advantageously, the metal of said metal coating is selected from the group consisting of nickel and copper.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.